This proposal seeks to investigate with a neonatal mouse model how various components in colostrum might alter the subsequent immune responsiveness of the nursing newborn. The responses to both parenteral and oral immunization will be measured. The possibility that hapten-protein conjugates in colostrum might induce a state of unresponsiveness or tolerance will be evaluated. In addition, the idea that colostral contact with a substance might alter the isotype distribution and molecular heterogeneity of the antibody response will be examined. Measurements to be made include the determination of antibody forming cells in various lymphoid tissues, the determination of serum antigen binding capacity, and isoelectric focusing of serum. The effects on immune responsiveness of both antibody and immune complexes in the colostrum will be studied. Another objective of this proposal is to assess the suggestion that the presence of antigen in the neonatal gut preferentially leads to a tolerant state maintained by suppressor cells. These possibilities will be evaluated by in vivo methods. Finally, the molecular properties of any suppressor substances will be examined in detail.